The invention pertains to a position sensor. A position sensor of this kind is known, for example, from EP 0386334 A2. The position of an adjustment drive unit, as described therein, is ascertained with two rotary potentiometers, each of which is linked by means of a gear to a toothed gear of a shaft. Both potentiometers are called xe2x80x9cendlessxe2x80x9d potentiometers which can execute an unlimited number of complete rotations (through 360xc2x0). The counting of the toothed gears is determined in that as one potentiometer performs a number n of complete rotations over a specified measuring range, the other potentiometer performs a number of complete rotations that differs by 1 (nxe2x88x921 or n+1). Then from the phase difference in the output signals of the two potentiometers, an absolute position signal can be determined.
Similar position sensors are also described in FR 2697081 A1, in DE 195 06 938 A1 and in the older, unpublished DE 198 49 554.
One method for high precision evaluation of two rotary position sensors is also found in DE 19747753 C1.
One specific area of application of the invention is a steering angle sensor in motor vehicles that determines the rotational position of the steering shaft which can perform several rotations. In principle, the invention can also be used on linear sensors in which a linearly moving gear rack is coupled to the toothed gear of the rotation position sensor.
The measuring accuracy depends, in part, on the precision of the toothing. Toothed gears and gear racks can have some play due to manufacturing tolerances, assembly tolerances pertaining to the shaft spacing, and also due to wear; in the more general sense, this is called the xe2x80x9cslip,xe2x80x9d and it is a source of measuring errors. During a reversal in the direction of direction of the rotary sensor, this error manifests as hysteresis error. An additional source of error in toothed gears and gear racks is lack of uniformity in the shape of the teeth as well as gaps between teeth, and also in irregularities in the distribution of the teeth, which, in turn, leads to errors in linearity in play-free engagement of the teeth. This also applies to unrounded toothed gears, which has as a result, that play is present in some regions of rotational adjustment and not in the case of other ranges of rotational adjustment. It must be taken into account that in most measurement applications, plastic toothed gears are being used.
It is the purpose of the invention to improve the position sensor of the initially mentioned type, so that measurement errors can generally be prevented, and at least reduced in magnitude.
The basic principle of the invention consists in providing a resilient, elastic tension of the toothed gear linked with the rotation position sensor, with respect to the toothing linked to the moving object.
The advantage achieved is that the shaft spacing between the driving and the driven shafts of the toothed gears or toothing of the position sensor is variable, and that the coupled toothing will always be optimally engaged. Thus, for many gear shapes (such as in an involute toothing, for example), at least one tooth of the toothed gear is engaged with its leading and its following flank with the corresponding flanks of the opposing teeth. Any play, slip in the sense discussed above, or dead play will thus be eliminated.
Preferably, the moving object is a rotary shaft and the toothing is a driven toothed gear connected to the shaft, with which two rotary position sensors are coupled by means of driven toothed gears. The rotary position sensors are seated in bearings and the bearings are pressed in the direction of the shaft by means of resilient, elastic tensioning bars.
An optimum force distribution is obtained when the shafts of rotation of the rotary position sensors and the shaft are located in a common plane.
An even better force compensation is obtained when the rotary position sensors are seated on a common bearing and the bearing is connected by means of resilient, elastic tension bars to an additional bearing which is braced against the driven toothed gear by means of an additional driven toothed gear. Then, preferably all driven toothed gears are tensioned in the direction of the midpoint of the driven toothed gear.
According to one refinement of the invention, the driven toothed gears are fixed in place by a position-retaining bearing and tensioning element with respect to the driven toothed gear, so that its rotational axis is kept in fixed position in the perimeter direction of the driven toothed gear and can only move radially with respect to the midpoint of the driven toothed gear.
Furthermore, the driven toothed gears, the bearings and the tension bar are preferably designed as a structural unit, which can slide nearly along a retaining plate.
According to an additional refinement of the invention, which can be used for general applications and not only in conjunction with the above properties, to increase the dependability of the position sensor on a new component, an error signal that is proportional to the measuring error of the position sensor is ascertained at the factory. This error signal and a specified limit value are stored. Then, during subsequent operation, a current error signal is determined and a check is made to determine whether the two error signals differ by more than the stored limit value, whereupon a warning signal will be generated.
To reduce computation time, the error signal ascertained at the factory is determined only for a specified rotational position, preferably the middle position, and the check of the current error signal likewise occurs only at this previously specified rotational position.